1. Field of the Invention
The present invention relates to an imaging apparatus, and more specifically to an imaging apparatus that forms a spectral image (spectroscopic image) in a predetermined wavelength range (band) by performing operation processing on an image signal that is obtained by imaging an object to be observed (an observation target) that is illuminated with light.
2. Description of the Related Art
In recent years, in the field of imaging apparatuses, such as electronic endoscopes using solid-state imaging devices, imaging apparatuses that perform spectral imaging (spectroscopoic imaging) by using narrow band-pass filters in combination have drawn attention. These imaging apparatuses (Narrow Band Imaging—NBI) have built-in narrow band-pass filters and the combination of the narrow band-pass filters is determined based on the spectral reflectance at digestive organs (gastric mucosa or the like). The imaging apparatus includes three narrow band-pass filters, which pass light in narrow (wavelength) bands, instead of a field-sequential-type (frame-sequential-type) rotation filter of R (red), G (green) and B (blue). In the imaging apparatus with the built-in narrow band filters, illumination light is sequentially output through the three narrow band-pass filters to illuminate an observation target, thereby obtaining three signals. Further, the obtained signals are processed in a manner similar to the processing performed in the case of R, G and B signals (RGB signals), while the degree of weighting is changed for each of the three signals. Accordingly, a spectral image is formed. If such a spectral image is used, it is possible to extract microstructures or the like from images of digestive organs, such as a stomach or large intestine, which could not be detected in the conventional method.
Besides the field-sequential-type endoscope using the narrow band-pass filters, a simultaneous-type endoscope that forms a spectral image by performing operation processing has been proposed. In the simultaneous-type endoscope, micromosaic color filters are arranged on a solid-state imaging device and the spectral image is formed by performing operation processing based on image signals obtained by using white light, as disclosed in Japanese Unexamined Patent Publication No. 2003-093336 and Yoichi Miyake, “Analysis and Evaluation of Digital Color Image”, University of Tokyo Press, 2000, pp. 148-153. In this method, the relationship between numerical data representing the color sensitivity characteristic for each of R, G and B and numerical data representing the spectral characteristic of a specific narrow band-pass filter is obtained as matrix data (a set of coefficients). Further, an operation is performed by using the matrix data and R, G and B signals. Accordingly, a spectral image signal representing an estimated spectral image that will be obtained by passing light through the narrow band-pass filter is obtained. If the spectral image is formed through such operations, it is not necessary to prepare a plurality of filters corresponding to desirable wavelength ranges. Further, it is not necessary to change the arrangement of the filters. Therefore, it is possible to prevent the size of the apparatus from becoming large. Further, it is possible to reduce the cost of the apparatus.
Meanwhile, in the field of observation of living organisms (organisms, living bodies or the like), as described above, a technique of observing fluorescence images of blood vessels, cancer cells or the like by marking them with a fluorescent marker (a fluorescent agent, a fluorescent reagent or the like) is used. Further, when such technique is used, application of the aforementioned spectral image technique to obtainment of the fluorescence images is considered. In observation of the living organisms, when absorption of light by water and hemoglobin contained in the living organisms is taken into consideration, it is desirable that the wavelength of the fluorescence and the wavelength of excitation light are approximately in the range of 600 to 1300 nm, and optionally in the range of 700 to 1300 nm, which is a near-infrared region.
However, the aforementioned apparatus, which obtains spectral images by performing operation processing, obtains spectral images in a visible light region by using R, G and B signals for ordinary images (color images). In other words, the apparatus is not designed to cope with the aforementioned wavelength ranges, i.e., 600 to 1300 nm and 700 to 1300 nm. Therefore, an additional optical system is needed to obtain a spectral image in the aforementioned wavelength range in addition to an ordinary image. Hence, there are problems that the size of the apparatus becomes large and that the cost significantly increases.